A document scanner is a device which converts a visible image such as a photograph, transparency or printed paper into an electronic form suitable for copying, storing or processing by a computer. Document scanners typically have a controlled source of light which is reflected off the surface of a document or is transmitted through a transparent document onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. In some configurations, the light source is a long tube providing a narrow band of light which extends to each edge of one dimension of the document or beyond the edges.
For electric discharge lamps such as fluorescent lamps, intensity is a function of power and temperature. The temperature of the vapor or gas and the phosphors indirectly affects intensity. When such a light source is first powered on, light intensity varies dynamically along the length of the tube until the overall temperature of the light source stabilizes. Document scanners using such a light source typically wait for some intensity stabilization before scanning the document.
Document scanners could simply wait open loop for a worst case lamp warm-up time before initiating a scan. For typical light sources, the required time is on the order of tens of seconds. In general, such a delay adds unnecessary additional time to every scan. Such a delay is particularly inappropriate if the lamp is already warm. In order to minimize overall scan times, some sort of light source stability sensing is needed.
In some document scanners, an area having a calibrated reflectivity is provided somewhere outside the document area. The calibrated reflectivity area is used to sense the intensity of the light source. Such configurations typically monitor light from a small section of the light source and that small section is typically near one end of a long tube. Scanning is initiated when the intensity of light from the small monitored section becomes stable. During the warm-up thermal transient period however, the intensity of the small section may be stabilized or more particularly, may be controlled, but other portions of the lamp may still be in transition. A mechanical analogy is a system with a mass on each end of a spring. Controlling the position of one mass does not immediately control the position for the mass at the other end of the spring. Likewise, for a tube with finite thermal mass, controlling the light intensity at one location does not immediately control the light intensity at remote locations. For high accuracy, a more accurate measure of overall light source intensity stability is needed.
In some document scanners, a uniform reflectivity target is used to calibrate the sensitivity of individual sensors in an array of photosensors. If sensor calibration is made while the intensity of the light source is still dynamically changing, an inaccurate sensor calibration may result. As a result, even though the intensity of the light source may be stable for most of the scan, the sensors will be inaccurate for the entire scan because of inaccurate initial calibration.
In addition, in some document scanners, a preview scan is used to adjust settings such as contrast and brightness for a final scan. If the preview scan is performed with a light source in which intensity is still dynamically changing and the final scan is performed with a light source with a stabilized intensity, settings based on the preview scan may not be appropriate. An accurate or at least a repeatable measure of overall light source intensity stability is needed to ensure that final exposure settings based on a preview scan meet a user's expectations.